1. Field of the Invention
The disclosures herein relate to an imaging apparatus provided with a photoelectric conversion film and an electron source array having electron sources arranged in matrix form wherein electrons are emitted from the electron source array during a video signal output period and a vertical blanking period.
2. Description of the Related Art
Research has been conducted for some time with respect to imaging apparatus that is provided with a photoelectric conversion film and an electron emission array having a matrix of electron emission sources, from which electrons are drawn out by an electric field without application of heat. This electron emission array has a plurality of Spindt-type emitters arranged in matrix form, which are opposed to the photoelectric conversion film across vacuum space. In such imaging device, holes that are generated and accumulated in the photoelectric conversion film in response to light arriving from an external source are read out by using electrons successively emitted from the Spindt-type emitter array, thereby producing a time sequence of video signals (see Patent Document 1).
An imaging apparatus of this type is known to suffer a capacitive residual image whose time constant is determined by a product of a static capacitance of the photoelectric conversion film and an equivalent resistance resulting from energy distribution of electrons emitted from the electron emission array.
When highly bright light enters the photoelectric conversion film, it may not be possible to read out, within a predetermined time period, all the holes generated and accumulated in the photoelectric conversion film by using electrons emitted from the electron emission array. In this case, a prominent residual image is created. This gives rise to a problem that an output image has degraded motion resolution and significantly reduced time resolution.
An imaging apparatus of this type is not provided with an electronic shutter function, which is adopted in a solid-state imaging device such as a CCD (Charge Coupled Device). When capturing an image of an object in motion, thus, it is impossible to compensate for degradation in motion resolution resulting from the accumulation, over a period of 1 field or 1 frame, of electric charge that is created by light entering the photoelectric conversion film.
Another problem is that an output image has flickers when images are taken under the lighting condition in which light is driven by a frequency lower than the field frequency or frame frequency of the imaging apparatus.
In order to overcome these problems, study has been conducted with respect to an imaging device in which the thickness of the photoelectric conversion film is increased, for example. In such imaging device, an increased thickness of the photoelectric conversion film serves to decrease the static capacitance, thereby reducing a capacitive residual image (see Non-Patent Document 1, for example).
Also, Patent Document 1 discloses forming a photoelectric conversion film on separate stripe-shape translucent electrodes and scanning the photoelectric conversion film formed on two adjacent translucent electrodes by use of two electron beams simultaneously emitted from the electron emission array.
In this imaging apparatus, the first electron beam is used to read holes generated and accumulated in the photoelectric conversion film to produce a video signal, and the second electron beam is used to remove the holes remaining in the photoelectric conversion film immediately after the scanning by the first electron beam. This serves to reduce a residual image.
Research has also been conducted with respect to another method that is different from the methods described above. In this method, voltage continues to be applied to a gate electrode during a residual charge sweeping period that follows immediately after an image signal output period, during which a pixel signal for a horizontal scan line is read (see Patent Document 2, for example).
Patent Document 2 further discloses applying a voltage to the gate electrode of a next horizontal scan line in an excessively-accumulated charge sweeping period following the residual charge sweeping period and setting the potential of cathode electrodes higher than the potential of a reference scan surface. This serves to remove the accumulated electric charge that is provided in excess of the amount readable within the video signal output period. With this arrangement, white saturation, smear, and resolution degradation resulting from imaging a highly bright object are prevented, thereby offering similar advantages to the use of an electronic shutter in a solid-state imaging device.
The method disclosed in Non-patent Document 1 increases the thickness of a photoelectric conversion film in an imaging device in which the photoelectric conversion film is opposed, across vacuum space, to an electron emission array having a matrix of electron emission sources. This reduces the static capacitance of the film, thereby suppressing a capacitive residual image.
Such method, however, can only reduce the static capacitance of the photoelectric conversion film. It is theoretically impossible to eliminate the static capacitance. A capacitive residual image thus sill occurs. Especially when the value of equivalent resistance resulting from energy distribution of electrons emitted from the electron emission array is large or when a large number of holes are generated and accumulated in the photoelectric conversion film in response to highly bright light, the occurrence of a capacitive residual image becomes prominent. This gives rise to a problem in that motion resolution is degraded in output images.
In the imaging apparatus using two electron scan beams for a photoelectric conversion film formed on separate stripe-shape translucent electrodes, it is necessary to shorten the intervals between adjacent translucent electrodes in order to reduce invalid imaging areas. There is thus an increase in static capacitance between the adjacent translucent electrodes. As a result, a portion of a video image generated by one of the electron beams is removed by the static capacitance existing between the adjacent translucent electrodes. This gives rise to the problem of lowered sensitivity and the like.
By the same token, the static resistance existing between adjacent translucent electrodes causes a video signal generated by one of the electron beams to be mixed with a signal that is generated upon removal of electric charge remaining in the photoelectric conversion film by the other electron beam immediately after the scan. This gives rise to a problem in that a pseudo signal comes into existence due to crosstalk.
In the imaging device described in Patent Document 2 in which voltage continues to be applied to a gate electrode during a residual charge sweeping period that follows immediately after an image signal output period during which a pixel signal for a horizontal scan line is read, the potential of the photoelectric conversion film on its scan side is reset to a potential close the cathode potential of electron emission sources immediately after the outputting of a video signal. Further, if the potential of the photoelectric conversion film on its scan side is close to the cathode potential, the speed of electrons in a direction perpendicular to the photoelectric conversion film decreases as the electrons come close to the photoelectric conversion film. The speed will be slow in the vicinity of the photoelectric conversion film.
Immediately before video signals adjacent to each other are output, on the other hand, the potential of the photoelectric conversion film on its scan side is significantly higher than the cathode potential. This is because holes generated by incident light are accumulated over a period of one field or one frame.
Because of this, electrons emitted for the purpose of removing residual holes in the photoelectric conversion film during the residual charge sweeping period mostly fail to reach the photoelectric conversion film at low potential immediately after the outputting of a video signal. The trajectories of these electrons are vent toward the portion of the photoelectric conversion film that is at significantly higher potential immediately before the outputting of a video signal. (This phenomenon will hereinafter be referred to as “vending”.) The electrons end up removing the holes accumulated there that constitute a video signal component. Accordingly, the method disclosed in Patent Document 2 suffers a problem in that the vending of electrons as described above limits the capacity of suppressing a residual image, and significantly lowers sensitivity.
In an electron emission array from which electrons are drawn out by an electric field, the amount of electrons emitted from electron emission sources may greatly vary from source to source on a given horizontal scan line. Also, the amount of electrons emitted from each electron emission source significantly varies with time.
As a result, the amount of holes existing immediately prior to scan that are neutralized by electrons having their trajectories vent by vending varies significantly from source to source, and varies significantly with time. An output image thus has sensitivity variation from pixel to pixel (i.e., variation in brightness from pixel to pixel in the image). Image quality thus noticeably drops.
In the imaging device described in Patent Document 2 in which a voltage is applied to the gate electrode of a next horizontal scan line in an excessively-accumulated charge sweeping period and the potential of cathode electrodes is set higher than the potential of a reference scan surface, it is only possible to prevent white saturation, smear occurrence, and resolution degradation. It is not possible to achieve the same operation as that of an electronic shutter used in a solid-state imaging device.
Electronic-shutter operation in a solid-state imaging device removes electric charge generated and accumulated in photodiodes in a partial period of a field period, for example, and reads electric charge generated and accumulated in the photodiodes in the remaining partial period. With this arrangement, motion resolution is improved. There has been a long-felt need for the provision of such electronic-shutter operation in the imaging apparatus as described above.
Accordingly, there is a need for a high-time-resolution imaging apparatus which is provided with an electronic shutter function, and can suppress occurrence of residual images without causing the lowering of sensitivity and image quality degradation resulting from sensitivity variation and/or crosstalk.    [Patent Document 1] Japanese Patent Application Publication No. 6-176704    [Patent Document 2] Japanese Patent Application Publication No. 2004-134144    [Non-patent Document 1] Honda et al., “Spindt-type FEA Image Sensor with Ultrahigh-sensitivity HARP Target,” Technical report of IEICE, ED2005-113, pp. 27-32, Japan, September 2005